1. Field of the Invention
The present invention generally relates to a color cathode ray tube and, more particularly, to the mounting of the shadow mask in the color cathode ray tube.
2. Description of the Prior Art
It is well known that a color cathode ray tube utilized as a display of, for example, a television receiver set employes a generally rectangular shadow mask assembly which is made of a perforated thin metallic plate or foil. The perforated thin metallic plate or foil has a multiplicity of minute circular apertures defined therein in a pattern coresponding to the triads of primary color elemental phosphor dots on the inner surface of the faceplate. Each of the triads correspond to the number of the primary colors.
When it comes to the manner by which the rectangular shadow mask is supported inside the evacuated envelope in the vicinity of the luminescent phosphor-deposited screen, a generally L-sectioned, generally rectangular rigid support frame is employed so as to intervene between the periphery of the shadow mask and the envelope.
More specifically, as shown in FIG. 1 of the accompanying drawings, the shadow mask is identified by 1. The envelope (not shown) comprises a funnel section (not shown) closed at one end by a generally cylindrical neck section (not shown) and at the other end by a generally rectangular faceplate 5. The rectangular faceplate 5 is generally in the form of a cup-shaped envelope cap including a screen plate 5a and a side wall 5b which is sealed to the funnel section. The screen plate 5a has an inner surface thereof deposited with triads of phosphor dots in a pattern corresponding to the pattern of the minute circular apertures in the shadow mask 1.
The generally L-shaped sectioned support frame is identified by 2 and has an axial flange 2a, which protrudes towards the screen plate 5a in a direction parallel to the longitudianl sense of the envelope. It further contains a radial flange 2b which extends generally perpendicular to the axial flange 2a and is retained in position within the envelope with the axial flange 2a thereof, connected to the side wall 5b of the faceplate 5. The shadow mask 1 is telescoped over and welded to the axial flange 2a of the support frame 2 while confronting, and spaced a predetermined distance inwardly from, the screen plate 5a.
As clearly shown in FIG. 1, the connection between the support frame 2 and the side wall 5b of the faceplate 5 is made by the use of a frame support structure 7 which comprises four stud pins 6, a bimetal piece 3 employed for each stud pin 6. It further comprises a leaf spring member 4 which is also employed for each stud pin 6. The stud pins 6 are embedded or otherwise permanently attached to the side wall 5b of the faceplate 5, and the four bimetal pieces 3 are rigidly secured or otherwise welded to the support frame 2 and distributed around the support frame 2. The leaf spring members 4 have one end 4a welded to the associated bimetal pieces 3. The other end 4b is formed with respective through-holes for the passage of the associated stud pins 6 therethrough for the connecting of the leaf spring members 4 to the side wall 5b of the faceplate 5.
When the color cathode ray tube, wherein the shadow mask 1 is supported within the envelope through the support frame 2 by way of the frame support structure 7 in the manner as hereinabove described, is operated, about 80% of the elctron beams, emitted from an electron gun assembly housed within the neck section of the envelope, impinge upon the shadow mask 1 and a portion of the support frame 2. Impingement of the electron beams upon the shadow mask and a portion of the support frame 2 results in an increase of temperature of the shadow mask mounting system as a whole. For example, in the case of the color cathode ray tubes generally used in commercially available television receiver sets, the amount of increase of the temperature of the shadow mask mounting system may be about 50.degree. at a central region of the shadow mask 1 and about 20.degree. at that portion of the support frame 2 and the frame support structure 7.
As is well known to those skilled in the art, increase of the temperature of the shadow mask support system as a whole brings about a displacement of the shadow mask 1 relative to the shadow mask support system as will now be discussed in details with particular reference to FIGS. 2 to 4.
Since the increase of temperature generally starts from the shadow mask 1, the temperature of the shadow mask 1 increases within a short length of time subsequent to the start of operation of the color cathode ray tube. Once the shadow mask 1 is consequently heated, the shadow mask 1 expands by reason of thermal effects so as to displace, as shown by the phantom line 1a in FIG. 2(a), in a direction close towards the screen plate 5a and generally parallel to the longitudinal axis Z of the evacuated envelope. With the subsequent passage of time, the support frame 2 is also heated and equally expands by reason of thermal effects to such an extent that, as shown by the respective phantom lines 1b and 2c in FIG. 2(b), both the shadow mask 1 and the support frame 2 displace not only in an X-axis direction perpendicular to the longitudinal axis Z of the evacuated envelope, but also in a Y-axis direction which is perpendicular to both of the longitudinal axis Z of the evacuated envelope and the X-axis direction.
In other words, when the shadow mask 1 is first heated, the shadow mask 1 is thermally expanded so as to result in an axial deformation or a `doming` in which a major portion of the shadow mask 1 protrudes towards the screen plate 5a as shown in FIG. 2(a). Subsequently, when the support frame 2 is heated by reason of thermal effects occasioned by the impingement of the electron beams and also by the heat transmission from the shadow mask 1, not only does the shadow mask 1 expand in a radial direction generally perpendicular to the longitudinal axis Z of the evacuated envelope, but the support frame 2 is also deformed in the radial direction as shown in FIG. 2(b). This results in a landing drift, which is a complex deformation including the dooming of the shadow mask 1.
In any event, once one or both of the shadow mask 1 and the support frame 2 are so displaced, the misalignment of the apertures in the shadow mask with the primary color elemental phosphor dots on the screen plate 5a occurs. This causes mislanding of the electron beams upon phosphor dots on the screen plate 5a. This in turn results in reduction of the color purity allowance and also in deterioration of the color purity.
As hereinabove discussed, the major cause of the landing drift is the thermal expansion of both of the support frame 2 and the shadow mask 1 in the X-axis and Y-axis directions. The bimetal pieces 3 used in the prior are mounting system discussed with reference to FIG. 1 are employed for the purpose of minimizing the landing drift (See U.S. Pat. No. 3,803,436.). Specifically, with the use of the bimetal pieces 3, the landing drift can be minimized by causing the shadow mask 1 and the support frame 2 to move towards respective positions, shown by the respective phantom lines 1c and 2d in FIG. 2(c), close to the screen plate 5a in a direction generally parallel to the longitudinal axis Z of the evacuated envelpe as the temperature of the mounting system as a whole increases.
The prior art attempt to minimize the landing drift places emphasis on the correction in a direction parallel to the longitudinal axis Z of the evacuated envelope. No substantial consideration has been paid to eventual effects brought about by the difference in thermal expansion between the support frame 2 and the frame support structure 7, because, where the support frame 2 is made of iron and the leaf spring members 4 are made of stainless steel, the difference in coefficient of thermal expansion is about 1.5.times.10.sup.-6 /.degree. C. Further, this occurs because, if the amount of increase of the temperature of one or both of the support frame 2 and the leaf spring members 4 is assumed to be 20.degree. and the effective length of each leaf spring member 4 is assumed to be 50 mm, the amount of displacement of the support frame 2 will be about 1.5 micrometers, which is tolerable.
However, when material of low coefficient of thermal expansion such as a metallic alloy containing, as a principle component, iron and nickel, is employed as a material for the shadow mask 1 and the sopport frame 2 for the purpose of minimizing the dooming of the shadow mask 1, the use of the leaf spring members 4 made of stainless steel will eventually result in a displacement of the support frame 2 of about 12 micrometers, or of about 30 micrometers when the ambient temperature subsequently elevates, under a condition identical with that described above.
In such case, the actual amount of displacement varies depending on the manner by which the support frame 2 is fitted to the side wall 5b of the faceplate 5. However, if the frame support structure 7 is so structured and so configured that, as shown in FIG. 3, the leaf spring members 4 may be oriented in the same direction around the periphery of the support frame 2, the displacement of the support frame 2 in a direction, shown by C, circumferentially thereof, which is occasioned by the elevation of the temperature, results in a twist of the support frame 2 and, hence, the shadow mask 1 in a direction about the longitudinal axis Z of the evacuated envelope as shown by the arrow A in FIG. 3.
If in order to minimize the twist of the support frame two of the leaf spring members 4 which are, on respective lateral sides of the support frame 2 adjacent the shorter sides of the rectangular shape of the support frame 2, are oriented in the same direction as shown in FIG. 4, e.g., downwards as viewed in FIG. 4, the difference of elongation between the support frame 2 and the frame support structure 7 will result in a shift of the support frame 2 and, hence, the shadow mask 1 not only in a direction shown by B, e.g. downwards as viewed in FIG. 4, in a plane perpendicular to the longitudinal axis Z of the evacuated envelope and containing the X- and Y-axes, but also in a direction parallel to the longitudianl axis Z of the evacuated envelope.
As discussed above, tthe displacement of only the support frame 2 is accompanied by a corresponding displacement of the shadow mask 1 which in turn brings about the reduction in color purity of the color cathode ray tube.
In general, in the case of the high resolution color cathode ray tube of 21 inch, 0.3 dot pitch model, the tolerance of color brightness is about 25 micrometers and the tolerance of color purity is about 40 micro-meters. Therefore, even though other affecting factors are taken into consideration, the amount of color misalignment which would result from the thermal expansion of the support frame 2 relative to the frame support structure 7 has to be restricted to 5 micrometers or smaller. Since the movement of the shadow mask 1 in the plane containing the X- and Y-axes is projected on the phosphor-deposited screen on the inner surface of the screen plate 5a, the difference in coefficient of thermal expansion between the support frame 2 and the frame support structure 7 must be 2.times.10.sup.-6 /.degree. C. or smaller. This allows for the amount of movement of the shadow mask 1 projected on the phosphor-deposited screen to be restricted to 5 micrometers or smaller.
It is pointed out that, if in the construction shown in FIG. 4 the leaf spring members 4 are made of the same material as that for the support frame 2, that is, the material of low coefficient of thermal expansion such as the Fe-Ni alloy, the displacement resulting from the difference in thermal expansion might be substantially eliminated. However, the material of a low coefficient of thermal expansion utilizing the Fe-Ni alloy tends to exhibit a low elasticity and will loose the elasticity when heated, as is occasioned during the manufacture of the color cathode ray tube. Therefore, once some or all of the leaf spring members 4 made of such material as hereinabove described loose the elasticity, they will no longer serve to retain the shadow mask assembly in position within the evacuated envelope, particularly, within the space delimited by the side wall 5b of the faceplate 5.